Environmental Engineering Reference
In-Depth Information
Consequently, these collectors are considerably easier to produce and maintain for
long periods than vacuum fl at-plate collectors. The shape of the glass tubes enables
them to withstand outside air pressure better so that metal rods are not necessary
for support.
In the enclosed glass tube of a vacuum tube collector is a fl ag absorber, which is a
fl at metal absorber that has a heat pipe integrated in the middle (Figure 6.7 left). A
lightly vaporizing medium like methanol is enclosed in this heat pipe. If it vaporizes
due to the heat of the sun, the vapour rises upwards. The heat pipe sticks out of the
glass tube at the top end. This is where the condenser, which condenses the heat
medium again, is located. In the process, it transfers the heat energy over a heat
exchanger to the water fl owing through. After condensation the once-more liquid
medium in the heat pipe fl ows downwards. For maximum functionality the tubes
should be mounted with a certain minimum tilt.
Figure 6.7 Vacuum-tube collectors. Left: Collector with heat pipe. Right: Tubes with direct fl ow.
Illustrations: Viessmann Werke.
Vacuum-tube collectors with passing heat carrier pipes (Figure 6.7 right) are also
available. With this system the heat carrier liquid fl ows directly through the collec-
tor. A heat exchanger is then not required and the collector does not necessarily
have to be mounted at an angle.
Because hydrogen molecules are extremely small, atmospheric hydrogen cannot be
totally prevented from penetrating the vacuum. With time this also destroys the
vacuum. So-called getters, which can chemically bind the hydrogen that penetrates
the vacuum over a long period of time, are built into a collector to prevent this from
happening.
The advantage of vacuum-tube collectors is a very high energy yield, especially
during the cooler seasons of the year. Compared to one with normal fl at - plate
collectors, a solar system with vacuum-tube collectors requires less space for
the collector. The disadvantage is the substantially higher cost of the collector
(Figure 6.8 ).
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